1. Field of the Invention
The present invention relates to a method of producing a room impression and/or sound impression of an actually existing room or of a calculated room, wherein any monophonic, stereophonic or multichannel audio program can be used as the auditory program. The reproduction is effected preferably binaurally through headsets; however, the reproduction can also be carried out through loudspeakers. The present invention also relates to an electroacoustic apparatus for carrying out the method.
2. Description of the Related Art
Generally, any produced audio program contains the architectural or room acoustics present during the recording. However, in the previously known stereophonic reproduction methods, the acoustics could never be completely recognizably reproduced in its fine structure. During the reproduction, the listener could not recognize more than that the recording was created in a room with a certain reverberation. Only additional measures with appropriate electroacoustic apparatus were capable of producing better auditory conditions, so that the listener could also recognize the room of the program recording.
For example, a simulation of room-acoustic events which is true to the original can be carried out by folding any selected audio program with the binaural room impulse response, measured at a certain location of reception in a room. Binaural room impulse response is considered to be two impulse responses, wherein one impulse response is assigned to one ear and the other impulse response is assigned to the other ear. In accordance with findings from system theory, the room forms together with the reception characteristics of the human ear a linear causal two part system which is described in the time domain by the room impulse responses. The respective room impulse response is approximately the system response to a sound impulse whose duration is a period of the double upper limit frequency of the audio signal. Convolving any audio program with the binaural room impulse response results in the signal which is suitable for electroacoustic reproduction, wherein the signal is formed in such a way that, with correct sound reproduction at both ears of a listener, an auditory experience is created in the listener as it would be experienced by the same listener at the original listening location at which the actual room acoustic event takes place. As a result, it becomes impossible to the listener to differentiate as to whether the auditory experience perceived by the listener takes place at the location of the actual sound event or whether it is produced by the simulation method. If loudspeakers are used for reproduction instead of headsets, the transmission paths between the loudspeakers and the ears of the listener must be reproduced essentially in the same manner.
A simulation method of this type which unmistakably precisely simulates to the listener the time-related, spectral, spatial and dynamic sound field structures which actually exist at the original listening location, is extremely complicated, particularly as far as the technical apparatus required for the simulation is concerned. Generally, convolution is carried out in such a way that the audio signal and the room pulse responses are digitalized, the convolved signal is calculated in a computer and is converted back into the analog signal. The number of calculation steps depends on the duration of the impulse responses. For example, in the case of an audio signal bandwidth of 20 kHz, a sampling frequency of approximately 50 kHz and, thus, a sampling interval of 20 .mu.sec are necessary and, therefore, 10.sup.5 samples are required for a typical room impulse response duration of 2 sec and, when convolving an audio signal with this room impulse response, 5.times.10.sup.4 .times.10.sup.5 =5.times.10.sup.9 multiplications and additions must be carried cut per second. This means that the apparatus required for convolving with an audio signal must be extremely large, particularly if the entire sequence of the method is to be carried out in real time. Accordingly, the use of such a simulation method outside of the realm of research is inconceivable for reasons of economy and expense.
An electroacoustic arrangement for a simulation which is virtually true to the original of an auditory situation existing at a certain listening location, is described in Austrian Patent 394,650 for the reproduction of stereophonic binaural audio programs by means of headsets. The auditive truth to the original and also the correct localization of certain sound sources distributed in the room can be ensured by correctly presenting a sound, which was originally recorded for the stereophonic loudspeaker reproduction for a virtually true headset reproduction if, in addition to the directly arriving audio signals of the two channels on the left and right, additionally the room reflections of the listening room are imitated, however, with the room reflections being weighted with the head related transfer functions which are dependent on the direction. The integration of the head related transfer function over all spatial directions results in an approximately flat amplitude frequency response at the ear. Since such a complex reproduction is practically impossible, a simplified configuration must be used. In this significantly simplified configuration, only three different audio signals must be presented to each ear for ensuring a true listening event.
The simulation of room-acoustic events can be carried out very generally by means of a method as it is known, for example, from European application 0 505 949. In this method, a transfer function is simulated by means of a transfer function simulator. This transfer function simulator is equipped with sound sources arranged in an acoustic system, sound receiving units and units for measuring the acoustic transfer function. For measuring the acoustic transfer function, the multitude of possible different positions between two arbitrary points in the acoustic system may be taken into consideration. The simulator proper is characterized in that means for estimating the poles present in the existing transfer function are provided, wherein the AR coefficients which correspond to the physical poles of the acoustic system are estimated from the multitude of measured transfer functions, and the ARMA filters, which are composed of AR filters and filters, reproduce that which coincides from the multitude of measured acoustic transfer functions with the acoustic system. This extremely complicated method has the purpose of reproducing an acoustic transfer function as it is required for echo cancelling units, for anti-reverberation units, for the active wind noise compensation and also for sound localization. The simulation of the transfer characteristics is carried out by a signal processor. In the simulation method itself, the transfer function is simulated with little calculation effort in the consequently shortest possible calculation time.
After appropriate modifications, the simulation method just described could essentially also be used for realizing the true reproduction of room-acoustic events. However, it would be technically extremely complicated and too specific, so that for the useful and economical use of this method there is no particular interest.
The known fast convolution by means of discrete Fourier transformation also does not offer a suitable solution for an economical unit for the simulation of room-acoustic events. This is because of the time delay between source signal and convolved signal which is inherent to this method.